The effect would look like drawing a black line, yes? There are many ways to draw a line.

If it is a 1-pixel-wide line as you seem to describe, it would take a very long time to make any kind of impact on the underlying image. Just think how long the line would have to be on a 1920x1080 HD image.

And just think how difficult it would be to even see a 1-pixel line in the first place.